The present invention relates generally to apparatus for controlling the pointing direction of a dual-spin satellite, and more particularly, to a closed-loop feedback system which employs both position and rotation rate information to control the pointing position of the payload platform of the dual-spin satellite.
A dual-spin satellite is a stabilized, or unstabilized, orbiting device having a spinning portion or rotor, and a despun portion or platform. However, other configurations are possible wherein only an antenna is despun. The two portions are coupled together by a despun motor and bearing assembly. The platform or antenna is usually pointed at a predetermined position on the earth, or at another satellite, or the like. Accurate control of the pointing position is critical to satellite operation.
The pointing position of the dual-spin satellite may be controlled by controlling the relative rate and relative position of the payload platform with respect to the spinning rotor. Prior art satellite pointing control systems include an open-loop system which provides for operator control of motor torque commands applied to an orbiting satellite from an earth station. The operator manipulated the rotation rate of the payload platform with respect to the earth to a low level, after which a position control loop controls the pointing position of the platform.
Another system provides for a position control loop which is utilized when the pointing direction of the platform is within a predetermined angular extent (deadband), while fixed torque signals are applied outside the deadband. The angular extent of the deadband is based on the relative rotational rate between the payload platform and rotor. A third control system comprising a phase-locked loop has been utilized which phase locks an index pulse train to an inertial sensor pulse train which indicates a known position. For multiple index pulses, the system can lock an inertial pulse to any index pulse. The system allows for manual stepping to any desired pulse which provides the correct pointing direction.